THIS WORKS EASILY, THANKS ENTITY - I spent a lot of time trying to figure out exactly what Entity meant, but after about 3 hours of grinding in that very location I finally understand.
If anyone is confused like I might have been, make sure after you jump down the final time you follow through PAST Leons cutscene, sprint Far forward to cause the objective bubble in the lower right to trigger the "bleeding" of the spec op guys. I however never shot any spec ops, instead i ran back and simply kited the blood lusted zombies. Over time, you will notice (more than likely after your AI teamates have been killed) they may or may not come around the front of the train. Look in between where the train karts divide and fire at enemies that are spawning directly under the train cars you are looking through, it will cause them to come around the corner in herds. Also try to throw some of your grenades into the distance for more XP at the very beginging when the zombies are in normal mode.... After about 20mins or so the spawns will STOP charging around the front train. At that point, just run forward blasting as many as you can and just restart the whole process after you die. REMEMBER YOU SAVE XP EVEN IF YOU DIE.
If my rambling isn't clear enough feel free to throw me a PM, regardless test it for yourself you will all figure it out eventually.
PS. 75,000XP in about 1.5hrs so far